MOA-2012-BLG-505Lb: A SuperEarth probably in the Galactic bulge

MOA-2012-BLG-505Lb: A super-Earth mass planet probably in the Galactic bulge

Authors:
Nagakane et al

Abstract:

We report the discovery of a super-Earth mass planet in the microlensing event MOA-2012-BLG-505. This event has the second shortest event timescale of tE=10±1 days where the observed data show evidence of planetary companion. Our 15 minute high cadence survey observation schedule revealed the short subtle planetary signature. The system shows the well known close/wide degeneracy. The planet/host-star mass ratio is q=2.1×10−4 and the projected separation normalized by the Einstein radius is s = 1.1 or 0.9 for the wide and close solutions, respectively. We estimate the physical parameters of the system by using a Bayesian analysis and find that the lens consists of a super-Earth with a mass of 6.7+10.7−3.6M⊕ orbiting around a brown-dwarf or late M-dwarf host with a mass of 0.10+0.16−0.05M⊙ with a projected star-planet separation of 0.9+0.3−0.2AU. The system is at a distance of 7.2±1.1 kpc, i.e., it is likely to be in the Galactic bulge. The small angular Einstein radius (θE=0.12±0.02 mas) and short event timescale are typical for a low-mass lens in the Galactic bulge. Such low-mass planetary systems in the Bulge are rare because the detection efficiency of planets in short microlensing events is relatively low. This discovery may suggest that such low mass planetary systems are abundant in the Bulge and currently on-going high cadence survey programs will detect more such events and may reveal an abundance of such planetary systems.

Dynamics of tidally captured planets in the Galactic Center

Dynamics of tidally captured planets in the Galactic Center

Authors:

Trani et al

Abstract:

Recent observations suggest ongoing planet formation in the innermost parsec of the Galactic center (GC). The super-massive black hole (SMBH) might strip planets or planetary embryos from their parent star, bringing them close enough to be tidally disrupted. Photoevaporation by the ultraviolet field of young stars, combined with ongoing tidal disruption, could enhance the near-infrared luminosity of such starless planets, making their detection possible even with current facilities. In this paper, we investigate the chance of planet tidal captures by means of high-accuracy N-body simulations exploiting Mikkola's algorithmic regularization. We consider both planets lying in the clockwise (CW) disk and planets initially bound to the S-stars. We show that tidally captured planets remain on orbits close to those of their parent star. Moreover, the semi-major axis of the planet orbit can be predicted by simple analytic assumptions in the case of prograde orbits. We find that starless planets that were initially bound to CW disk stars have mild eccentricities and tend to remain in the CW disk. However, we speculate that angular momentum diffusion and scattering with other young stars in the CW disk might bring starless planets on low-angular momentum orbits. In contrast, planets initially bound to S-stars are captured by the SMBH on highly eccentric orbits, matching the orbital properties of the G1 and G2 clouds. Our predictions apply not only to planets but also to low-mass stars initially bound to the S-stars and tidally captured by the SMBH.

The microlensing rate and distribution of free-floating planets towards the Galactic bulge

The microlensing rate and distribution of free-floating planets towards the Galactic bulge

Authors:

Ban et al

Abstract:

Ground-based optical microlensing surveys have provided tantalising, if inconclusive, evidence for a significant population of free-floating planets (FFPs). Both ground and space-based facilities are being used and developed which will be able to probe the distrubution of FFPs with much better sensitivity. It is vital also to develop a high-precision microlensing simulation framework to evaluate the completeness of such surveys. We present the first signal-to-noise limited calculations of the FFP microlensing rate using the Besancon Galactic model. The microlensing distribution towards the Galactic centre is simulated for wide-area ground-based optical surveys such as OGLE or MOA, a wide-area ground-based near-IR survey, and a targeted space-based near-IR survey which could be undertaken with Euclid or WFIRST. We present a calculation framework for the computation of the optical and near-infrared microlensing rate and optical depth for simulated stellar catalogues which are signal-to-noise limited, and take account of extinction, unresolved stellar background light and finite source size effects, which can be significant for FFPs. We find that the global ground-based I-band yield over a central 200 deg^2 region covering the Galactic centre ranges from 20 Earth-mass FFPs year^-1 up to 3,500 year^-1 for Jupiter FFPs in the limit of 100% detection efficiency, and almost an order of magnitude larger for a K-band survey. For ground-based surveys we find that the inclusion of finite source and the unresolved background reveals a mass-dependent variation in the spatial distribution of FFPs. For a space-based H-band covering 2 deg^2, the yield depends on the target field but maximizes close to the Galactic centre with around 76 Earth through to 1,700 Jupiter FFPs year^-1. For near-IR space-based surveys the spatial distribution of FFPs is found to be largely insensitive to the FFP mass scale.

Investigating the free-floating planet mass by Euclid observations

Investigating the free-floating planet mass by Euclid observations

Authors:

Hamolli et al

Abstract:

The detection of anomalies in gravitational microlensing events is nowadays one of the main goals among the microlensing community. In the case of single-lens events, these anomalies can be caused by the finite source effects, that is when the source disk size is not negligible, and by the Earth rotation around the Sun (the so-called parallax effect). The finite source and parallax effects may help to define the mass of the lens, uniquely. Free-floating planets (FFPs) are extremely dim objects, and gravitational microlensing provides at present the exclusive method to investigate these bodies. In this work, making use of a synthetic population algorithm, we study the possibility of detecting the finite source and parallax effects in simulated microlensing events caused by FFPs towards the Galactic bulge, taking into consideration the capabilities of the space-based Euclid telescope. We find a significant efficiency for detecting the parallax effect in microlensing events with detectable finite source effect, that turns out to be about 51% for mass function index .

Is the Galactic Bulge Devoid of ExoPlanets?

Is the Galactic bulge devoid of planets?

Authors:

Penny et al

Abstract:

Using a sample of 31 microlensing exoplanet hosts, we investigate whether or not the distances to these systems conform to the Galactic distribution of planets expected from a model. We derive the expected distribution of distances from a simulated microlensing survey, correcting (roughly) for the dominant selection effects that affect the detection sensitivity to planets as a function of their distance, and compare with the observed distribution using Anderson-Darling (AD) hypothesis testing. We find that the AD test rejects the hypothesis that the observed sample is drawn from our model distribution of distances with p-value 5.0×10−4. Interestingly, we find that an AD test can not reject (p=0.18) the hypothesis that the observed sample is drawn from a model distribution where only disk stars host planets (i.e., a model where there are no bulge planets), though AD tests of the relative proper motion distributions indicate tension with this extreme hypothesis. Allowing the relative abundance of planets in the bulge to that in the disk, fb, to be a free parameter, we find that AD tests do not reject models where fb less than 0.54 for p greater than 0.01. We find that allowing for a dependence of planet occurrence rate on host mass and metallicity does not significantly change our results, and if we fix these dependencies to their respective trends as inferred from RV surveys, the limit only weakens to fb less than 0.7. We suspect that some of the distance estimates for nearby (Dl less than 2 kpc) lenses could be significantly in error. If this is indeed the case, removal of two of the potentially problematic hosts we identify would allow for fb less than 0.96. While we cannot conclusively infer fb given current observations, more homogeneous samples from on-going surveys that are easier to model will allow the measurement of fb using similar methods to those developed in this work. (abridged)

MOA 2011-BLG-028Lb: a Neptune-mass Microlensing Planet in the Galactic Bulge

MOA 2011-BLG-028Lb: a Neptune-mass Microlensing Planet in the Galactic Bulge

Authors:

Skowron et al

Abstract:

We present the discovery of a Neptune-mass planet orbiting a 0.8 +- 0.3 M_Sun star in the Galactic bulge. The planet manifested itself during the microlensing event MOA 2011-BLG-028/OGLE-2011-BLG-0203 as a low-mass companion to the lens star. The analysis of the light curve provides the measurement of the mass ratio: (1.2 +- 0.2) x 10^-4, which indicates the mass of the planet to be 12-60 Earth masses. The lensing system is located at 7.3 +- 0.7 kpc away from the Earth near the direction to Baade's Window. The projected separation of the planet, at the time of the microlensing event, was 3.1-5.2 AU. Although the "microlens parallax" effect is not detected in the light curve of this event, preventing the actual mass measurement, the uncertainties of mass and distance estimation are narrowed by the measurement of the source star proper motion on the OGLE-III images spanning eight years, and by the low amount of blended light seen, proving that the host star cannot be too bright and massive. We also discuss the inclusion of undetected parallax and orbital motion effects into the models, and their influence onto the final physical parameters estimates.

Is the G2 Cloud a Rogue Planetary Embryo?

Signatures of planets and protoplanets in the Galactic center: a clue to understand the G2 cloud?

Authors:

Mapelli et al

Abstract:

Several hundred young stars lie in the innermost parsec of our Galaxy. The super-massive black hole (SMBH) might capture planets orbiting these stars, and bring them onto nearly radial orbits. The same fate might occur to planetary embryos (PEs), i.e. protoplanets born from gravitational instabilities in protoplanetary disks. In this paper, we investigate the emission properties of rogue planets and PEs in the Galactic center. In particular, we study the effects of photoevaporation, caused by the ultraviolet background. Rogue planets can hardly be detected by current or forthcoming facilities, unless they are tidally disrupted and accrete onto the SMBH. In contrast, photoevaporation of PEs (especially if the PE is being tidally stripped) might lead to a recombination rate as high as ~10^45 s^-1, corresponding to a Brackett-gamma luminosity ~10^31 erg s^-1, very similar to the observed luminosity of the dusty object G2. We critically discuss the possibility that G2 is a rogue PE, and the major uncertainties of this model.